Unless otherwise indicated herein, approaches described in this section are not prior art to the claims listed below and are not admitted to be prior art by inclusion in this section.
Due to the rapid evolution of wireless technology and the increasing demand for higher data rates, modulation schemes employed in wireless communications are becoming ever more complicated. Complex modulation schemes generate transmit signals with high peak-to-average ratios (PAPR) which degrade the efficiency of radio-frequency (RF) power amplifiers (PA). The situation becomes severer when more bands are required in Long Term Evolution (LTE) services. RF PAs tend to function less efficiently when they are configured to cover a wider frequency range. Moreover, services based on LTE Advanced (LTE-A) with carrier aggregation (CA) command more complicated RF front-end components that are accompanied with more insertion losses pushing RF PAs to output higher power which may be infeasible for conventional RF PAs with fixed supply voltage.
To address this issue, an envelope tracking (ET) technique has been proposed. With ET, the envelope of the actual modulated RF signal is tracked. By instantaneously adjusting the PA supply voltage according to the envelope of the modulated signal, ET can effectively reduce the power consumption of PA and, therefore, significantly improve the overall system efficiency. Recognizing the ability of ET in efficiency improvement, many smartphone vendors have adopted ET in their smartphones. Besides gallium arsenide (GaAs) PAs, ET is also useful in improving linearity and efficiency of complementary metal-oxide-semiconductor (CMOS) PAs that are widely used in WiFi applications.
ET calibration in factories typically considers electrical characteristics such as error vector magnitude (EVM), adjacent channel leakage ratio (ACLR), receive-band noise (RXBN) and power efficiency. However, due to the limited calibration time in production lines in a factory, it is difficult to calibrate good or optimal ET parameters. For instance, in production lines, the total time for measuring transmitter (TX) performance for one test condition is about 1 second. Specifically, a test computer controls a mobile device (e.g., a smartphone) to transmit TX signals to a test instrument, and the test computer obtains TX measurement results from the test instrument. It usually takes a long time for ET factory calibration to find optimal ET parameters (e.g., constant gain mapping, non-constant gain mapping, ET-TX path delay and so on) because of tradeoff among main performance indexes in terms of EVM, ACLR, RXBN and power efficiency. However, a desirable calibration time would be too long and unacceptable for production lines in the factory setting.